Radio frequency (RF) transmitters are found in many one-way and two-way communication devices, such as portable communication devices, (cellular telephones), personal digital assistants (PDAs) and other communication devices. An RF transmitter must transmit using whatever communication methodology is dictated by the particular communication system within which it is operating. For example, communication methodologies typically include amplitude modulation, frequency modulation, phase modulation, or a combination of these. In a typical GSM mobile communication system using narrowband TDMA technology, a GMSK modulation scheme supplies a very clean phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator.
In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used, thus allowing efficient transmission of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Other transmission standards, such as that employed in IS-136, however, use a modulation scheme in which the transmitted signal contains both a PM component and an amplitude modulated (AM) component. Standards such as these increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, existing GSM modulation schemes are not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a signal containing a PM component and an AM component, a highly linear power amplifier is required. Unfortunately, highly linear power amplifiers are very inefficient, thus consuming significantly more power than a non-linear power amplifier and drastically reducing the life of the battery or other power source.
This condition is further complicated because transmitters typically employed in GSM communication systems transmit in bursts and must be able to control the ramp-up of the transmit power as well as have a high degree of control over the output power level over a wide power range. In GSM this power control is typically performed using a closed feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the control port of the power amplifier.
When attempting to include a PM component and an AM component in a GSM type modulation system, the power control loop will attenuate the amplitude variations present in the signal in an attempt to maintain a constant output power. In such an arrangement, the power control loop tends to cancel the AM portion of the signal.
In such systems in which transmit signals contain both PM and AM components, the output power can be controlled by applying a predetermined control voltage to the power amplifier. Unfortunately, this requires the use of a highly linear, and therefore comparatively inefficient, power amplifier. In non-burst transmission systems the output power may be controlled by a feedback loop having a time-constant that is very low compared to the time-constant of the amplitude variations of the modulator. Another known method to control the output power is to “pre-distort” the modulated signal in such a way that the power control loop will cancel the effect of the pre-distortion. In such a method, the amplitude information is passed through a transfer function that is the inverse of the power control loop transfer function. Unfortunately, these methods are costly and inefficient.
Known multi-mode transmitter architectures require multiple variable elements, which are chosen depending upon the desired transmit mode. These architectures are complex, unreliable, require periodic calibration, and cannot support multiple transmission standards without significant adjustments to the supporting analog and digital circuitry.
Further, in those transmission standards in which the signal to be transmitted by the power amplifier contains both phase modulation and amplitude modulation, unless the power amplifier is very linear, it may distort the combined transmission signal by causing undesirable AM to PM conversion. This conversion is detrimental to the transmit signal and can require the use of a costly and inefficient linear power amplifier.
Further still, in transmission systems in which a combined AM and PM signal is used in a power control loop having a wide range of power output levels and to meet strict GSM/enhanced data rates for GSM evolution (EDGE) spectral emissions limitations, a high level of precision is typically required when converting the amplitude modulated portion of the transmit signal from the digital domain to the analog domain. This typically requires a digital-to-analog converter (DAC) having 12 bit resolution with a sampling rate of approximately 4 MHz. Such a DAC is costly to implement, both with respect to area on the circuit and power consumption.